In many conventional gas fired appliances, such as boilers, clothes dryer, ovens and the like, it is customary to provide heat by igniting gas emanating from a main burner. Commonly, gas flows through the main burner when the device is activated, the gas being ignited by a nearby pilot flame which is constantly burning. Recognizing the inefficiency and danger of a constantly burning pilot flame, automatic ignition systems which rely upon heat from a resistive element to ignite the main burner have been substituted for constantly burning pilot systems, the resistive element being energized only when the device calls for heat. In such systems, it is known to employ a silicon carbide resistive element having a negative temperature characteristic (i.e. the resistance of silicon carbide decreases with increasing temperature) as the igniter. One such prior art system is described in U.S. Pat. No. 3,282,324, the contents of which are hereby incorporated by reference in their entirety.
In the system disclosed in U.S. Pat. No. 3,282,324, a solenoid activated gas valve is employed, the solenoid winding being in a circuit with the igniter element. Because silicon carbide has a negative temperature characteristic, when the device calls for heat, current flow through the igniter heats the igniter thereby dropping its resistance. This continues until current flow through the circuit incorporating the solenoid winding increases sufficiently to energize the solenoid and open the gas valve.
To close the gas valve in the event of a flameout, the system includes a circuit which deenergizes the igniter element after the gas valve is opened. The igniter element then operates as a heat detector, the gas valve being closed if current flow through the igniter element drop below a predetermined value considered indicative of a sufficient drop in temperature to confirm a flameout.
It will be apparent that in both the ignition and heat detection modes, the system disclosed in U.S. Pat. No. 3,282,324 is based on the assumption that current flow through the igniter element, and hence its resistance, is an accurate indication of the igniter element temperature. Unfortunately, this assumption ignores the reality that the resistance/temperature characteristic for different silicon carbide igniter elements varies from one igniter element to the next. That is, one igniter element might display one temperature at a particular resistance, while another igniter element might display a quite different temperature at that resistance. Accordingly, relying on a predetermined igniter element resistance level as an indication that its temperature is sufficient to ignite gas results in a potentially inaccurate system. Furthermore, the time required for the system discussed in the patent to open and close the gas valve is relatively slow.
Another prior art system is disclosed in U.S. Pat. No. 3,933,419, the contents of which are also hereby incorporated by reference in their entirety. In the system disclosed in this patent, a heat sensing plate comprised of a magnetic alloy having a predetermined Curie temperature is employed to determine when the temperature of the igniter element is sufficient to ignite gas. In particular, the heat sensing plate exhibits magnetic properties at room temperature which are sufficient to attract a permanent magnet in a circuit operatively connected to the gas valve. As long as the permanent magnet is attracted to the plate, the valve remains closed. However, as the plate is heated by current flow through the igniter element, its Curie temperature is eventually reached at which point the plate loses its ability to attract the magnet. As a result, the magnet moves away from the plate under the urging of a spring whereupon the gas valve is opened. As usual, shortly after the gas valve is opened, the igniter element is deenergized, and as long as the heat of the flame keeps the temperature of the igniter element sufficiently high to maintain the plate above its Curie temperature, the gas valve remains open. In the event of a flameout, the temperature in the vicinity of the igniter element drops, and hence the temperature of the heat sensing plate also drops. As a result, the plate again acquires magnetic properties sufficient to attract the permanent magnet and close the gas valve.
It will be apparent, therefore, that the system disclosed in U.S. Pat. No. 3,933,419 relies upon the point at which the magnetic plate loses its magnetic attractability as an accurate indication of the temperature of the plate, and hence of the igniter element. This system is, therefore, based on the questionable assumption that the permanent magnet and heat sensing plate can be manufactured in commercial quantities with sufficiently uniform magnetic properties to insure that the gas valve will not be prematurely opened or closed. Furthermore, the response time of this system is also relatively slow.